Aligning a derailleur hanger: do you really need the pricey tool?

The tool threads in to the hanger and pushes the alignment face square against the face of the hanger. Then there's the lever arm and a reference guide showing you where it is in alignment with the wheel.

Pretty simple tool, I think accuracy will make this one a tough one to replicate, though. Those hangers can be finicky. I'm interested to see your results if you do try it.

Can you do it with a homemade tool? Sure, do a search for one, there are a few floating around.

Is it worth it considering the price vs. accuracy of the professional tool? I don't think so. If you are dealing with 10 speed clusters, eyeballing it can be difficult, but not impossible. The proper tool works on any bike, and quickly at that.

If you are deadset on not spending money, shove the long end of a 5mm hex into the derailleur mounting bolt, hold it in with force, and bend. It is, at best, just as good. At worst? You damage the derailleur.

Been thinking the same. Someone on here said that a normal threaded axle has the same thread as the derailleur so can use that as the pivot.
But I was in Lowes just yesterday and was looking at square tubing. The tubing was anywhere from $14 to $25 depending on steel or aluminum.
I'm sure I can find it cheaper but it's looking like building a half assed one is going to run close to the $25 mark. Wondering if it isn't worth paying the $59 for the proper one.

My local bike coop has one. Almost every major city has a bike coop. I pay 7 bucks an hour to use all their tools.

But still, for the occasional hanger tweak, I clamp a 12 inch steel ruler on the hanger and eyeball the ruler against the cassette at 12 o'clock and 9 o'clock, peering at the ruler from above and behind the bike while its on the stand. Then I tweak with a crescent wrench.

It normally takes 3 to 5 adjustments and no longer than 5 minutes to do the job.

I've used the front axle approach with success with drivetrains up to 8-spd rear(remove front wheel, set QR aside, and thread the whole wheel into the derailleur hangar. Use rim for leverage and reference points -> can do a good deal of fine tuning ) . Just haven't needed to try it on a 9 or 10-spd rear in several years of riding them, so investment not worth it In my case.

I've mentally argued this myself since my GT Sanctions replacement hanger is $55 a pop. We all know how finicky this hangers are after a crash and how annoying it gets. Once I was chatting w/ another rider on the trailhead while he's making adjustments with his derailleur alignment tool so I had a good look. My DIY version was guna cost around $30+ w/ some work/assembly required and not nearly as neat & user-friendly as the Park Tool. So I got one shipped-free - Park Tool DAG-2 Derailleur Hanger Alignment Gauge - AAWYEAH Bikes and Bicycle Parts

You can definitely go cheap as what other folks here recommend. I've gotten lucky w/ a pair of pliers once. The Park tool is quick, easy and accurate. For me, peace of mind is priceless.

I made my own tool using a length of 50x25 densified wood, an M10x75 fine pitch bolt, 10 or so M10 washers and 2 nuts.
Drilled a 10mm hole in the wood, greased and inserted the bolt with 1 washer, stacked a bunch more washers on the other side and locked the two nuts against each other to allow it to swing on the bolt.
Could add some kind of gauge to it, but I can get the job done easily enough with a combination square.
Sorry for the lack of pictures, down to a lack of posts.

You'll be glad to have the DAG2. I think you'll find, as I did, that you'll be reaching for that tool first when there's anything even a little bit hinky with a bike's shifting; far more often than you expect. Any other playing, adjusting, tuning you might try is wasted if the hanger's tweaked. For me, checking hanger alignment has become part of the normal pre-race or pre-big-ride prep I do on the family's bikes.